A tax year is a particular period of twelve months which is used by the government as a basis for calculating taxes and for organizing its finances and accounts. In Britain, the tax year begins on April 6th and ends on April 5th. In the United States, the tax year begins on January 1st and ends on December 31st. the period of 12 months in which income is calculated for paying taxes. The tax year begins on April 6th in Britain, and January 1st in the US